編譯錯誤使用boost :: future .then（）

我想使用boost :: future .then（）函數。該片段取自Boost 1.54.0 thread synchronisation documentation 編譯錯誤使用boost :: future .then（）

#include <string> 
#include <boost/thread/future.hpp> 
int main() { 
    boost::future<int> f1 = boost::async([]() { return 123; }); 
    boost::future<std::string> f2 = f1.then([](boost::future<int> f)->std::string { 
              int x = f.get(); 
              return ("Done" + std::to_string(x)); 
              }); 
}

設置：
Ubuntu的13.04
克++版本克++（Ubuntu的4.8.1-2ubuntu1〜13.04）4.8.1
升壓版本1.54.0

命令行：

g++ then_test.cc -std=c++0x -DBOOST_THREAD_VERSION=4 -I  /home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost -L /home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/stage/lib -static -lboost_thread-mt -lboost_date_time-mt -lboost_system-mt -lpthread

錯誤：

g++ then_test.cc -std=c++0x -DBOOST_THREAD_VERSION=4 -I /home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost -L /home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/stage/lib -static -lboost_thread-mt -lboost_date_time-mt -lboost_system-mt -lpthread 
then_test.cc: In function ‘int main()’: 
then_test.cc:10:44: error: no matching function for call to ‘boost::future<int>::then(main()::__lambda1)’ 
              }); 
              ^
then_test.cc:10:44: note: candidates are: 
In file included from then_test.cc:2:0: 
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp:1598:9: note: template<class F> boost::future<typename boost::result_of<F(boost::future<R>&)>::type> boost::future<R>::then(F&&) [with F = F; R = int] 
     then(BOOST_THREAD_FWD_REF(F) func); 
     ^
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp:1598:9: note: template argument deduction/substitution failed: 
In file included from then_test.cc:2:0: 
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp: In substitution of ‘template<class F> boost::future<typename boost::result_of<F(boost::future<R>&)>::type> boost::future<R>::then(F&&) [with F = F; R = int] [with F = main()::__lambda1]’: 
then_test.cc:10:44: required from here 
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp:62:29: error: no type named ‘type’ in ‘struct boost::result_of<main()::__lambda1(boost::future<int>&)>’ 
#define BOOST_THREAD_FUTURE future 
          ^
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp:3840:3: note: in expansion of macro ‘BOOST_THREAD_FUTURE’ 
    BOOST_THREAD_FUTURE<R>::then(BOOST_THREAD_FWD_REF(F) func) 
^
In file included from then_test.cc:2:0: 
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp:1601:9: note: template<class F> boost::future<typename boost::result_of<F(boost::future<R>&)>::type> boost::future<R>::then(boost::launch, F&&) [with F = F; R = int] 
     then(launch policy, BOOST_THREAD_FWD_REF(F) func); 
     ^
/home/prakash/maidsafe/MaidSafe/build/boost_1_54_0/src/boost/boost/thread/future.hpp:1601:9: note: template argument deduction/substitution failed: 
then_test.cc:10:44: note: cannot convert ‘<lambda closure object>main()::__lambda1{}’ (type ‘main()::__lambda1’) to type ‘boost::launch’ 
              });

請讓我知道如果我在這裏失去了一些東西。

來源

2013-10-11 prakash

Boost.ResultOf默認不使用'decltype'，因此不能使用lambda表達式。 '#define BOOST_RESULT_OF_USE_DECLTYPE'在包含Boost標頭之前，最好在項目範圍內。 – Xeo

通過引用傳遞未來.then（）修復了gcc 4.8 & clang上的編譯問題。

對於windows和gcc 4.7我們還需要定義BOOST_RESULT_OF_USE_DECLTYPE。（根據Xeo's comment）。對於gcc 4.8 &鐺它似乎已經可用。

boost::future<std::string> f2 = f1.then([](boost::future<int>& f)->std::string { 
                  ^

來源

2013-10-11 12:06:41 prakash

with boost 1.55它需要右值參考，即將來 && f – Sumant

編譯錯誤使用boost :: future .then（）

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